A gas chromatograph (GC) is an instrument that is used to separate volatile components of a sample. A GC is typically used in combination with a suitable detector (e.g. a mass spectrometer) to analyze, identify, and/or measure individual components within the sample.
FIG. 1 shows a high-level schematic diagram of a portion of an exemplary gas chromatograph (GC) 10. The GC 10 includes a GC inlet 11 connected to a column 20. A carrier gas 18 flows into an inlet body 12 via an input path 22. A liquid sample 15 is introduced into the inlet body 12 by a needle 14. The inlet body 12 is a heated chamber for evaporating the liquid sample 15. The inlet body 12 is covered by a septum 16, which is a flexible material through which the needle 14 is inserted to inject the liquid sample 15. A portion of the carrier gas 18 flows underneath the septum 16 to prevent contamination of the current analysis by material left on the septum by previous injections. This flow exits through the septum purge 17. The components of the sample 15 separate within a column 20 based on their physical characteristics and are carried by the carrier gas 18 to a detector (not shown).
The septum 16 is made of a flexible material (such as rubber) which is capable of being pierced by the needle 14. The flexible material also conforms around the needle 14 to create a seal, so that the contents of the inlet body 12 do not escape through any gaps between the septum 16 and the needle 14. Once the needle 14 is removed, the flexible material of the septum 16 rejoins and closes up the hole that was created by the needle 14 when it pierced the septum 16. Some septa have pre-formed holes for inserting the needle.
However, the septum 16 is prone to developing leaks because its ability to reseal the holes formed by the needle degrades over time and repeated use. Any leakage from the septum 16 is problematic because it may invalidate a GC measurement.
Various methods have been used in the past to try to prevent or manage septum leaks. For example, some users schedule a change of the septum on a regular basis, e.g. once a week. However, septum leaks are random so a replacement schedule only gives a margin of protection.
Furthermore, changing the septum alone will not prevent or cure leaks elsewhere within the GC inlet 11. For example, another possible source of leaks is around a column ferrule, which connects the column 20 to the inlet body 12. Such a leak could be the result of temperature cycling or may happen during column maintenance. A leak may even be caused by a broken column. Other flow disruptions that invalidate a GC measurement—such as clogs or other obstructions—may also exist.
Therefore, there remains a need for an improved method and apparatus for detecting leaks and other flow disruptions within the GC.